Compression check results
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From: Princeton, NJ
Originally Posted by r0tor
The line ends at 9.7 kgf/sq cm and 300 rpm. The line with the above slope would hit 10.0 kgf/sq cm at 315 rpm. It could never reach more then 10.0 beacause the engines compression ratio is 10:1. Atmospheric pressure is 1 kgf/sq cm and working with the ideal gas law you have P2 = (v1/v2)*P1 or the compression ratio*P1 or 10.
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From: yourI'mgirl
r0tor: I'm pretty certain that the 10:1 compression is purely based on changes of the geometry of the volume and has nothing to do with all the other factors that you mentioned. Its not reported as 10:1 because that's what they measured with all the sealing, thermal expansion, other effects etc. Its reported as 10:1 because volume at TDC is 1/10th the volume at BTC.
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From: Whitby Ont
I don't know which Service Manual you guys are looking at,but mine has 2 lines on the pressure v engine speed graph----line A is standard pressure and line B is minimum pressure.At 300rpm the minimum compression pressure is about 7.8 kgf/cm2.(See section 01-10-7).
In any event the procedure for doing the compression test requires you to get the engine upto normal operating temperature and then WAIT 10 mins to allow the exhaust ports to cool off.The techs I have seen do not always do this and if they don't you get a lower pressure reading.
Last point is if you run with 5W30 --10W40 you will have sufficient compression in this engine.
In any event the procedure for doing the compression test requires you to get the engine upto normal operating temperature and then WAIT 10 mins to allow the exhaust ports to cool off.The techs I have seen do not always do this and if they don't you get a lower pressure reading.
Last point is if you run with 5W30 --10W40 you will have sufficient compression in this engine.
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From: Princeton, NJ
Originally Posted by two rotors
I don't know which Service Manual you guys are looking at,but mine has 2 lines on the pressure v engine speed graph----line A is standard pressure and line B is minimum pressure.At 300rpm the minimum compression pressure is about 7.8 kgf/cm2.(See section 01-10-7).
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From: Whitby Ont
Originally Posted by Charles R. Hill
Two, if you look at my initial response I first asked if there was any proof that the engine was warmed up. I had similar issues with the tech at my dealer's service department having, most likely, not warmed up the engine nor used the WDS to also measure engine cranking speed. As a result I got numbers in the range of 6.2-6.5 or so without benefit of the rpm values. The service manual I have is the c.d. manual, version 2.0 from 9/2003. What I find so compelling during these conversations is that when our focus turns a bit so as to also include the Mazda tech's methodology, there is ALWAYS a serious doubt as to their having followed proper procedures. I strongly believe that this forum provides more accurate information via discussions such as these than do the dealerships/techs or Mazda, themselves.
What is your opinon regarding the use of 5W20 as specified by Mazda with regard to crankng compression. Plus, has it been mentioned yet that dirty oil can cause low readings and poor engine performance?
What is your opinon regarding the use of 5W20 as specified by Mazda with regard to crankng compression. Plus, has it been mentioned yet that dirty oil can cause low readings and poor engine performance?
I presume Mazda(Japan) tested the engine with 5W30 oil because that is specified in all markets except North America---but I have really no way of knowing for sure.
It is not absolutely necessary to use the WDS,the earlier electronic tool does the job as well,but you can't make a nice print out of the results.(In fact they use the same pressure transducer.)Any tool which gives the compression pressure without simultaneous rpm measurement is not really good enough especially at the limit.
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From: New Jersey
Originally Posted by rx8-rus
Talking about compression specs we always refer to the original 2004 service manual. Have somebody seen updated manual for 2007 model? They come with the new starter from factory, and thus all have higher cranking speed. Manual is probably updated with the new data to reflect this.
As for the method of testing affecting the results, the dealership I used to go to ran my first compression test while my engine was cold. Here are those results - https://www.rx8club.com/showpost.php...9&postcount=23.. I don't know if letting the exhaust ports cool off for 10 minutes, as two rotors mentioned, would affect the results similarly.
why is it everyone is worried about the engine compression test results and not the engine vacuum test results?
The vacuum test (which is what Mazda did for the recall) is performed at idle and also tests the seals of the engine but while the engine is actually at operating temp/ up to speed / and with the MOP in service?
The vacuum test (which is what Mazda did for the recall) is performed at idle and also tests the seals of the engine but while the engine is actually at operating temp/ up to speed / and with the MOP in service?
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I got a new engine recently. So far it runs well. Dealer replaced one of the ignition coils also. So far (500 miles on the new engine) car runs well. I didn't notice any change in MPG. Car feels a little stronger in low RPM range and has more linear acceleration over 6000 RPM. But it is just feeling, I didn't do any instrumentation measurements yet.
On the other forum I found a post from mechanic who said that if compression exceeds 7 you won't have problems with hard start or dying on idle. But overall power output will be reduced. So if you have a car made in 2003-2005 and it is still under warranty, I recommend to check compression even if you did not notice problems yet.
On the other forum I found a post from mechanic who said that if compression exceeds 7 you won't have problems with hard start or dying on idle. But overall power output will be reduced. So if you have a car made in 2003-2005 and it is still under warranty, I recommend to check compression even if you did not notice problems yet.
Last edited by rx8-rus; May 17, 2007 at 02:12 PM.
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From: Princeton, NJ
After the first oil change in a new engine I decided to do a compression check. Results are 8.4-8.5 in all chambers at 270 RPM. I also decided to use only synthetic 5w30 oil in it (Valvoline). Hopefully this engine will last until the end of life for the rest of a car.
The other thing to do is to pour a couple of oz of auto tranny fluid into the LIM and crank the motor with the plugs out and the ESS unpluged.
Keep adding fluid until it starts to spray out of the plug holes and then put the plugs back in and let it sit over night.
Plug the ESS back in and start it in the morning.
The resultant white cloud is quite impressive, but it will release almost all of the carbon from the seal grooves and LIM.
My motor turned 10.1 at 230 RPM after doing that. It settled down to a more reasonable 9.1 after a few hours of normal operation.
So after running this thru my 8 the oil test began. I am in GA and temp were above 85F everyday. First 5w-20 which gave me ok engine performance, fairly smooth idle, and about 190-200 miles per tank. I changed to 5w-30 and re-fueled using the same fuel. The engine performance increased slightly, idle was smoother and got 215-230 miles per tank. Finally I tried 10w-30 with BG MOA in the oil. Upon startup my car was so quiet and smooth I could not tell it was runnning. On the test drive it seemed to have much more power and it was 93F out that day. I drove the car all last night and it still just screams and seems to have more torque. I am on track to get 280 miles out of this tankful as well.
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bg44k is the best, for what it does.
if you do the atf trick be sure your cat is not attached? Will not ATF hurt the cat?
It is a good old time treatment.
Kennyfc--pleasure speaking with you last w/e at the meet. I too tried 10W30 some time back and it did smooth things out but i also noticed a little power lost--but that was butt dino and we know how accuate that is.
OD
if you do the atf trick be sure your cat is not attached? Will not ATF hurt the cat?
It is a good old time treatment.
Kennyfc--pleasure speaking with you last w/e at the meet. I too tried 10W30 some time back and it did smooth things out but i also noticed a little power lost--but that was butt dino and we know how accuate that is.
OD
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Laid back in the pocket
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